Pressure sensor

ABSTRACT

A pressure sensor includes a sensor element, and a resin package member that holds the sensor element. The sensor element is constructed by a semiconductor, and is capable of externally outputting an electric signal in accordance with strain generated when force is applied thereto. The sensor element is directly adhered to the package member via an adhesive layer that has Young&#39;s modulus in a range between 2.45×10 3  Pa and 2.06×10 4  Pa. Further the adhesive layer has a thickness equal to or more than 110 μm. Accordingly, the pressure sensor effectively restricts a variation in a sensor characteristic due to a thermal change.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-45186 filed on Feb. 21, 2002.

FIELD OF THE INVENTION

[0002] The present invention relates to a pressure sensor in which a sensor element is mounted in a resin package. In the pressure sensor, the sensor element is directly adhered to the resin package through an adhesive layer.

BACKGROUND OF THE INVENTION

[0003] Conventionally, this kind of pressure sensors has been proposed in JP-P2000-356561A. According to the pressure sensor, a sensor element constituted by a semiconductor is adhered to a lead frame and mounted in a resin package member via the lead frame. Further, the pressure sensor is provided with a non-contact space between the package member and an area, where the sensor element is mounted, in the lead frame. The sensor is thereby restricted from thermal hysteresis. Here, the thermal hysteresis is a phenomenon in which a sensor characteristic changes from an initial characteristic due to a cooling-heating cycle with repeat of high and low temperatures. However, in a viewpoint of cost reduction, it has been recently desired that the sensor element is directly adhered to the package member with an adhesive without the lead frame.

[0004] When the sensor element is mounted in the package member via the lead frame, surfaces of a lead portion and an island portion in the lead frame are located on the same plane. Therefore, a level gap (step portion) is formed between surfaces of the lead portion and the sensor element mounted on the island portion, and is necessary to bond the land portion and the sensor element with a wire. Here, to upgrade wire-bonding performance, the surfaces of the sensor element and the lead portion are preferably on the same plane.

[0005] Removal of the island portion under the sensor therefore leads to flexible designing of the component layout to possibly set the two bonded surfaces on the same plane. It has been hence desired that the sensor is adhered on the package member with an adhesive layer without using the lead frame.

[0006] However, when the above direct adhesion of the sensor element to the package member is performed, the thermal hysteresis is readily generated by a difference of thermal expansion coefficients among the sensor, the adhesive and the package member.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to restrict variation of a sensor characteristic due to thermal fluctuation in a pressure sensor in which a sensor element is directly adhered to a package member via an adhesive layer.

[0008] To achieve the above and other objects, a pressure sensor is provided with a sensor element which is directly adhered to a package member via an adhesive layer that has Young's modulus in a range between 2.45×10³ Pa and 2.06×10⁴ Pa. Therefore, the thermal fluctuation of the sensor characteristic can be effectively restricted.

[0009] Preferably an adhesive layer contains therein a plurality of beads. Therefore the adhesive layer having the Young's modulus in a range between 2.45×10³ Pa and 2.06×10⁴ Pa can be readily formed. When an adhesive layer has a thickness of equal to or more than 110 μm, the advantage for restricting the variation of the sensor characteristic can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

[0011]FIG. 1 is a sectional view of a pressure sensor of an embodiment of the present invention;

[0012]FIG. 2 is a plan view of the pressure sensor of the embodiment taken from arrow II in FIG. 1;

[0013]FIG. 3 is a graph showing relationship between a Young's modulus of an adhesive layer and a change amount of a sensor characteristic;

[0014]FIG. 4 is a graph showing relationship between an adhesive layer thickness and a change amount of the sensor characteristic; and

[0015]FIG. 5 is an enlarged sectional view of an adhesive layer containing beads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] In an embodiment of the present invention, a pressure sensor is typically used for an atmospheric pressure sensor S1 for detecting the atmosphere pressure. Referring to FIG. 1, a sensor element 10 of the pressure sensor S1 includes a semiconductor substrate 11 such as a silicon substrate and a seat 12 that is made of glass. The substrate 11 has a cavity, so that a pressure reference chamber 13 is provided between the substrate 11 and the seat 12 bonded to each other. Accordingly, the sensor element 10 is provided with the pressure reference chamber 13, and constructs an absolute-pressure detecting element which detects force or pressure applied to an outer surface of its own.

[0017] The sensor element 10 forms a diaphragm 14 in the substrate 11 at a portion corresponding to the chamber 13. On the diaphragm 14, gauge resistors (not shown) are formed to constitute a bridge circuit.

[0018] On the substrate 11, an amplification circuit (not shown) is formed to be electrically connected with the bridge circuit. When external force acts on the diaphragm 14, the diaphragm 14 is strained and deformed. An electric signal (power voltage) from the bridge circuit due to the strain or the deformation of the diaphragm 14 is amplified to output it in the amplification circuit.

[0019] The sensor element 10 is contained in a concavity 21 formed within the package member 20 while the seat 12 of the sensor element 10 is directly adhered on the bottom of the concavity 21 via an adhesive layer 30. Accordingly, the sensor element 10 is held in and fixed to the package member 20

[0020] The package member 20 is formed by molding resin such as polyphenylene sulfide (PPS). The adhesive layer 30, made of resin, has a Young's modulus in a range between 2.45×10³ Pa and 2.06×10⁴ Pa (between 2.5×10⁻⁴ kgf/mm² and 2.1×10³ ⁻kgf/mm²). The adhesive layer 30 can use a silicon-based adhesive composed of silicon resin. The thickness of the adhesive layer 30 is preferably not less than 110 μm.

[0021] Here, a lead frame 40 is integrally inserted and mold in the package member 20 and externally extends from the concavity 21 of the package member 20.

[0022] The lead frame 40 is formed by stamping a plate made of, for example, 42-alloy (42% Ni—Fe alloy). The lead frame 40 is provided with a land portion 41 exposed around the concavity 21 within the package member 20, and a lead portion 42 extending outward from the land portion 41 to penetrate through the package member 20. In the embodiment, as illustrated in FIG. 2, the eight lead portions 42 are provided totally in the right and left parts, for example.

[0023] The land portion 41 of the lead frame 40 is electrically connected to the sensor element 10 by wire bonding using wires 50 made of a metal such as alumina and gold. Wire-connecting surfaces of the sensor element 10 and the land portion 41 are positioned on the substantially same plane, so that the wire bonding can be readily accurately performed.

[0024] The lead portion 42 of the lead frame 40 is connected with an external connecting member (not shown) in the outside of the package member 20. As shown in FIG. 2, the package member 20 is provided with a circuit chip 60 which is electrically connected with the sensor element 10 and the land 41, by the wires 50.

[0025] The circuit chip 60 is capable of adjusting an output signal from the sensor element 10. Specifically, the output signal (output voltage) from the sensor element 10 is adjusted by the circuit chip 60 via a wire 50 and externally outputted from a lead frame 40 via a wire 50.

[0026] As shown in FIG. 1, the surface of the sensor element 10, the surface of the semiconductor substrate 11 in the sensor element 10, and connecting parts between the land portions 41 of the lead frame 40 and the wires 50 are covered and sealed to be protected by a gel member 70.

[0027] A plastic lid 80 is connected to the package member 20, for instance by adhesion at the side of the concavity. The lid 80 seals the sensor element 10, the circuit chip 60 and the wire 50 contained in the package member 20.

[0028] The lid 80 is provided with a pressure inlet 81 communicating with the atmosphere. Through the pressure inlet 81 pressure from the atmosphere, that is measuring object, is introduced into a space defined by the lid 80 and the package member 20.

[0029] The diaphragm 14 is stressed and deformed due to pressure difference between the introduced atmosphere and the reference chamber 13 in the sensor element 10. The sensor element 10 thus outputs an electric signal as voltage based on the stressed deformation. The output voltage is adjusted by the circuit chip 60 via the wire 50 and is externally outputted.

[0030] The above pressure sensor S1 is manufactured as follows. First, the lead frame 40 is fixed in a mold that has an inner shape corresponding to the package member 20 and the inner shape of the mold is filled with resin to form the package member 20 integrated with the lead frame 40. Next, the circuit chip 60 is mounted on the package member 20, the sensor element 10 is fixed to the package member 20 using the adhesive layer 30, and the circuit chip 60 and the sensor element 10 are bonded by the wire bonding using the wires 50. Thereafter the gel member 70 is provided, and the lid 80 is assembled to the package member 20. In addition, an entire surface of the sensor element 10 facing the adhesive layer 30 can be adhered to the package member 20 through the adhesive layer 30 so that a stable Young's modulus of the adhesive layer 30 can be obtained.

[0031] In the present invention, the sensor element 10 is directly bonded to the package member 20 by the adhesive layer 30. A main characteristic of the embodiment is that the adhesive layer 30 has a Young's modulus in a range between 2.45×10³ Pa and 2.06×10⁴ Pa (between 2.5×10⁻⁴ kgf/mm² and 2.1×10⁻³ kgf/mm²). Maintaining the Young's modulus of the adhesive layer in the above range is to restrict the variation of the sensor characteristic due to the fluctuation of the temperature, and is to improve the performance of the pressure sensor S1. The basis of defining the Young's modulus of the adhesive layer 30 is explained as below.

[0032] The thermal hysteresis is a difference in a sensor output at the room temperature (for instance 25°C.) between an initial state and a state after a thermal-history (e.g., after being hated to 120°C.). That is, thermal hysteresis is a phenomenon in which the sensor output fluctuates at the room temperature.

[0033] The phenomenon is generated from varying of an output characteristic of the sensor element 10 since a creep strain due to the thermal-history in the package member 20 is applied to the sensor element 10 via the adhesive layer 30. Reduction of the Young's modulus in the adhesive layer 30 therefore relieves a strain from the package member 20 to the sensor element 10 to less affect the sensor characteristic.

[0034] The above relation is formulated as a formula (1) below. Here, a force F(p) is applied to the sensor element 10 by the package member 20 while a force F(t) is applied to the sensor element 10 by the adhesive layer 30. A voltage Vout is initially outputted as an initial voltage of the sensor element 10 while a voltage Vout′ is outputted as an output of the sensor element 10 which has experienced a thermal-history to vary.

Vout′=Vout×F(p)/F(t)  (Formula 1)

[0035] Here, the force F(p) depends on material of the package member 20, such as a resinous material that composes the package member 20, while the force F(t) depends on the thickness of the adhesive 30 and the Young's modulus of material of the adhesive 30.

[0036] A change amount (change ratio) of the sensor characteristic due to the fluctuating of the temperature is substantially equivalent to a difference between the Vout′ and the Vout. The relationship between the Young's modulus of the adhesive layer 30 and the change amount of the sensor characteristic is shown in FIG. 3 by a finite element method (FEM) analysis, for example.

[0037] A vertical axis is the change amount of the sensor characteristic while a lateral axis is the Young's modulus of the adhesive layer 30 in FIG. 3. The change amount of the sensor characteristic is the one at the room temperature after a thermal-history (thermal cycling) in a range between−40° C. and 120° C. The change amount is indicated with a percentage relative to a full scale (FS) of an output voltage of the pressure sensor S1. Here, since the change amount requests high accuracy for an in-vehicle use, an allowed criterion must be within one percent of the full scale.

[0038] As shown in FIG. 3, to achieve the change amount within one percent of the full scale, the adhesive layer 30 should have a Young's modulus in a range between 2.45×10³ Pa and 2.06×10⁴ Pa (between 2.5×10⁻⁴ kgf/mm² and 2.1×10⁻³ kgf/mm²). Here, in the example illustrated in FIG. 3, material of the package member 20 is PPS while material of the adhesive layer 30 is a silicon-based adhesive and the adhesive layer 30 is 110 μm thick. According to an analysis and experiments by the inventors, other cases, where different materials are used, also have relationship similar to the trend illustrated in FIG. 3.

[0039] The stress applied from the package member 20 to the sensor element 10 decreases as the thickness of the adhesive layer 30 becomes thicker. The change amount of the sensor characteristic relative to the thickness of the adhesive layer 30 is thereby analyzed with a FEM analysis as shown in FIG. 4.

[0040] The instance illustrated in FIG. 4 is obtained under the most severe condition in which the Young's modulus of the adhesive layer 30 is 2.1×10⁻³ kgf/mm² at which the change is only just in the one percent change in FIG. 3. The result of FIG. 4 exhibits that the adhesive layer of at least 110 μm thickness ensures the change range within one percent of the full scale even if the Young's modulus has the most severe value.

[0041] In other words, when an adhesive layer that has a Young's modulus in the above predetermined range and a thickness of more than 110 μm is used, performance of pressure sensor S1 is improved. This pressure sensor S1 effectively restricts the change of the sensor characteristic due to thermal fluctuation.

[0042] It is not so easy to ensure as thick as 110 μm adhesive layer by using an adhesive whose Young's modulus is as low as in this embodiment. Therefore, resin beads 31 can be contained in the adhesive layer 30 to ensure the thickness as shown in FIG. 5. Because an excessive addition of the resin beans 31 to the adhesive layer 30 increases the Young's modulus, the addition amount of the resin beans 31 is adjusted to maintain the suitable Young's modulus of the adhesive layer 30. Here, resin beans 31 can be made of a material different from the adhesive layer.

[0043] A silicon-based hot-cured adhesive (brand name TSE 322) manufactured by Toshiba Silicon Corporation can be used, for example, in the embodiment. The resin beads 31 are inserted in the adhesive (TSE 322) and the adhesive is then hot-cured, so that a Young's modulus of the adhesive layer 30 can be set in the range between 1.5×10⁻³ kgf/mm² and 2.1×10⁻³ kgf/mm^(2.) 

What is claimed is:
 1. A pressure sensor comprising: a sensor element constructed by a semiconductor, the sensor element being strained in accordance with an applied external force, and outputting an electric signal based on the strain; a package member made of resin, that is disposed to support the sensor element; and an adhesive layer through which the sensor element is directly adhered to the package member, wherein the adhesive layer has a Young's modulus in a range between 2.45×10³ Pa and 2.06×10⁴ Pa.
 2. A pressure sensor according to claim 1, wherein the adhesive layer has therein a plurality of beads.
 3. A pressure sensor according to claim 1, wherein the adhesive layer has a thickness equal to or more than 110 μm.
 4. A pressure sensor according to claim 1, wherein the adhesive layer is formed by a silicon-based adhesive.
 5. A pressure sensor according to claim 1, wherein: the sensor element has a surface facing the adhesive layer, and all the surface of the sensor element is adhered to the package member through the adhesive layer. 